A function in a vehicle is known with which it is possible to lock or unlock a vehicle door or start up an engine without using a mechanical key. The function is called a “smart entry & start system” or a “smart key system”.
In a smart key system, communications are carried out between a portable device (key) and an Electronic Control Unit (ECU) or a Body Control Module (BCM) mounted in a vehicle. In a case where communications have been established between the portable device and the ECU or BCM, locking or unlocking a door, or starting up the engine can be accomplished.
A portable device is known that waits for a wake-up signal with first receiving sensitivity lower than second receiving sensitivity, and prevents erroneous starting up that may otherwise occur due to a signal other than the wake-up signal, i.e., due to a signal that is noise for the portable device (for example, see Japanese Laid-Open Patent Application No. 2011-234260).